The flaps can occupy a position in which they substantially extend the wall of the afterburner channel, and a position in which they are inclined relative to the axis of the afterburner channel so as to increase thrust.
The flaps, which are metal in the prior art, come into contact with the high-temperature gas leaving the afterburner of the turbojet, and as a result they have a relative short lifetime that could be increased by making the flaps out of a material that withstands high temperatures better, e.g. a ceramic-matrix composite material (CMC), which material would also reduce the weight of the flaps without substantially increasing their cost.
However, mounting hot flaps of CMC material in a convergent exhaust nozzle of the above-mentioned type poses the problem of sealing between the flaps at the peripheral gasket arranged at the outlet from the afterburner channel: the flaps of CMC material are several times thicker than metal flaps, and as a result of their layered sheet structure, they cannot have a small radius of curvature in the join zones between adjacent flaps. This results in relatively large leakage sections in the join zones through which afterburner gas can escape, thereby reducing the amount of thrust provided in afterburner mode.
In addition, it would not be satisfactory to equip the flaps of CMC material with metal portions at their ends that co-operate with the above-mentioned peripheral gasket because of the differential thermal expansion between the metal portions and the composite material.